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FnIO MODBUS Adapter NA-9171, NA-9173
FnIO S-Series
Copyright(C) CREVIS Co.,Ltd Support +82-31-899-4599 URL : www.crevis.co.kr
6. MODBUS INTERFACE
6.1.
MODBUS Transmission Mode
Two different serial transmission modes are defined: The RTU mode and the ASCII mode. It defines the bit
contents of message fields transmitted serially on the line. It determines how information is packed into the
message fields and decoded.
6.1.1.
RTU Transmission Mode
When devices communicate on a MODBUS serial line using the RTU (Remote Terminal Unit) mode, each 8–bit
byte in a message contains two 4–bit hexadecimal characters. The main advantage of this mode is that its greater
character density allows better data throughput than ASCII mode for the same baud rate. Each message must be
transmitted in a continuous stream of characters.
Start
Address
Function
Data
CRC Check
End
≥
3.5 chars
1 char
1 char
Up to 252 chars
2 chars
≥
3.5 chars
6.1.2.
ASCII Transmission Mode
When devices are setup to communicate on a MODBUS serial line using ASCII (American Standard Code for
Information Interchange) mode, each 8–bit byte in a message is sent as two ASCII characters. This mode is used
when the physical communication link or the capabilities of the device does not allow the conformance with RTU
mode requirement regarding timers management.
Start
Address
Function
Data
CRC Check
End
1 char
2 chars
2 chars
Up to 252 chars
2 chars
2 chars CR,LF
6.2.
Supported MODBUS Function Codes
Function Code
Function
Description
Unicast / Broadcast
1 (0x01)
Read Coils
Read output bit
Unicast
2 (0x02)
Read Discrete Inputs
Read input bit
Unicast
3 (0x03)
Read Holding Registers
Read output word
Unicast
4 (0x04)
Read Input Registers
Read input word
Unicast
5 (0x05)
Write Single Coil
Write one bit output
Unicast / Broadcast
6 (0x06)
Write Single Register
Write one word output
Unicast / Broadcast
8 (0x08)
Diagnostics (Serial Line only)
Read diagnostic register
Unicast
15 (0x0F)
Write Multiple Coils
Write a number of output bits
Unicast / Broadcast
16 (0x10)
Write Multiple registers
Write a number of output words
Unicast / Broadcast
23 (0x17)
Read / Write Multiple register
Read a number of input words /
Write a number of output words
Unicast
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Refer to MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1a
